1. Field of the Invention
The present invention relates to an electric brake for generating a braking force according to a torque of a motor. More specifically, the present invention relates to an electric brake provided with a parking brake function.
2. Description of Related Art
As an example of an electric brake, there can be mentioned an electric brake comprising a caliper having provided therein a piston, an electric motor and a rotary-linear motion conversion mechanism for enabling rotation of the motor to be converted to a linear motion and transmitted to the piston. A thrust is given to the piston according to rotation of a rotor of the motor, to thereby press a brake pad against a disc rotor, thus generating a braking force. Normally, in this electric brake, a driver's force applied to a brake pedal, or a stroke of the brake pedal, is detected by a sensor, and rotation (an angle of rotation) of the motor is controlled based on a value detected by the sensor, to thereby obtain a desired braking force.
Recently, various attempts have been made to increase an advantage of such an electric brake by providing therein a parking brake (PKB) mechanism.
For example, there exists an electric brake having a parking brake mechanism, in which when an actuator coil of the parking brake mechanism is deenergized, an actuator plunger is retracted under force of a compression spring, so that a pivot arm having an engaging pawl is placed in an initial upright position and engages a tooth portion of a ratchet, to thereby place the parking brake in an active state. When the actuator coil is energized, the actuator plunger is advanced (an extension stroke), to thereby effect disengagement of the engaging pawl of the pivot arm from the tooth portion of the ratchet and thus release the parking brake (see FIGS. 6 to 8 of Japanese Patent Application Public Disclosure No. 2003-42199).
Further, there is disclosed another example of an electric brake having a parking brake mechanism. In this example, to apply the parking brake, an actuator plunger is advanced (an extension stroke) by energizing an actuator coil, to thereby engage an engaging pawl with a tooth portion of a ratchet against a force of a tension spring. To release the parking brake, the electric motor is operated in a direction for braking while the actuator coil is deenergized, to thereby loosen the engagement between the engaging pawl and the tooth portion of the ratchet. As a result, the engaging pawl is pulled by the tension spring and disengaged from the tooth portion of the ratchet, thus releasing the parking brake (see FIGS. 9 and 10 of Japanese Patent Application Public Disclosure No. 2003-42199).
In these prior art techniques, if the actuator or the electric motor malfunctions, or the supply of current to the coil becomes impossible due to malfunctioning of a component for supplying a current to the actuator or the motor when the parking brake mechanism is operated (when the parking brake is applied), the engaging pawl is continuously held on the ratchet. In this case, the parking brake cannot be released if no countermeasure is taken, and the vehicle cannot be moved even as a temporary action. Therefore, it is desirable to solve the above-mentioned problem of the prior art techniques.
In view of the above, the present invention has been made. It is an object of the present invention to provide an electric brake having a parking brake function, which enables the parking brake to be readily released even in the case of malfunctioning of the actuator or the motor, or a failure of supply of current to the actuator or the motor.